Numerical investigation of fan performance in a forced draft air-cooled steam condenser

[摘要] ENGLISH ABSTRACT: Forced draft air-cooled steam condensers (ACSCs) consisting of multiple fan units are usedin direct cooled power plants to condense steam in a closed steam cycle. Axial flow fanslocated below an A-frame configuration of finned tube heat exchanger bundles, force ambientair through the system. In so doing, heat from the condensing steam is rejected to theenvironment via the finned tubes. The performance of an air-cooled system is proportional tothe air mass flow rate and the temperature difference between the finned tubes and theambient air. A variation in either will directly affect the efficiency of the steam turbines.Air flow distortions at the fan inlet caused by structures, wind and other fans may result in asignificant reduction in flow rate as well as fan blade vibration. This phenomenon has anadverse affect on the cooling capacity of an ACSC, and consequently turbine performance,due to a decrease in air mass flow rate. In this study the effect of inlet flow distortions on fanperformance (i.e. flow rate and fan shaft power) in an ACSC is numerically investigated bymodelling a section (or sector) of such a system using the commercial computational fluiddynamics (CFD) code, FLUENT. Fan performance at different platform heights, andcorresponding different degrees of inlet flow distortions, is investigated.The performance of two types of axial flow fans are also compared. The two fans have thesame diameter, number of blades and rotational speed, but feature different blade designs,and hub-tip-ratios of respectively 0.153 and 0.4. A fan model based on blade element theory,better known as an actuator disc model, is used to numerically model the fans.Previous experimental studies have shown that a solid walkway installed along the edge orperiphery of an ACSC platform can significantly increase the flow rate through the fanssituated along the platform edge. The effects of such a walkway, and other windscreens onfan performance, are numerically investigated.Numerical predictions correlate with earlier experimental results: the flow rate and fan shaftpower are decreased by inlet flow distortions. It was found that the fan with a hub-tip-ratio of0.4 was less affected by these flow distortions. The addition of a walkway increased the flowrate through the edge fan by up to 48 %. It is furthermore shown that wind effects can only beaccurately modelled if the entire ACSC is considered.

[发布机构] Stellenbosch University